The research proposed herein deals with the regulation of cholesterol metabolism and in particular its rate-limiting enzyme 3-hydroxy-3 -methylglutaryl coenzyme A reductase. The enzyme has been purified to homogeneity and preliminary evidence indicates that it is associated with lipids. A detailed analysis is intended to reveal the identity of these lipids and the stoichiometry of their association with the enzyme. Reconstitution of the purified enzyme with these lipids containing a fluorescent probe should elucidate the nature of this binding and its relation to activity. By using the antibody prepared to the purified enzyme its level within the liver can be monitored. Studies will be conducted to explain whether the decreased activity of this enzyme and hence cholesterol biosynthesis upon intake of dietary cholesterol is due to enhanced degradation of the enzyme or its inactivation. Furthermore, an investigation of the rapid turnover of the enzyme is intended to explain the sequence of events causal to its rapid degradation. It is not clear whether the intestinal activity of HMG-CoA reductase and cholesterol biosynthesis are regulated directly by cholesterol or by its combined action with bile acids. By developing a method to obtain ideal cells we intend to ascertain the effects of cholesterol, its derivatives and lipoproteins in order to determine whether intestinal cholesterogenesis is sensitive to these effectors.